feat: optimize Docker startup with Next.js standalone output and proper migrations

- Add `output: standalone` to next.config.js for faster cold start
- Rebuild runner stage around standalone bundle (node server.js instead of pnpm start)
- Replace prisma db push with prisma migrate deploy (proper migration workflow)
- Remove npx/pnpm at runtime, use direct binary paths
- Add HOSTNAME=0.0.0.0 for standalone server to listen on all interfaces
- Fix next.config.js not copied in builder stage
- Update README: pnpm instead of yarn, correct ports, full env vars documentation

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

next.config.js

@@ -1,5 +1,6 @@
 /** @type {import('next').NextConfig} */
 const nextConfig = {
+  output: "standalone",
   webpack: (config) => {
     config.resolve.fallback = {
       ...config.resolve.fallback,